Move module-util.h to src/shared/ and load_module() to libshared

Move module-util.h to src/shared/ and load_module() to libshared

Unfortunately this needs libshared to link to libkmod. Before it was linked
into systemd-udevd, udevadm, and systemd each seperately. On most systems this
doesn't make much difference, because at least systemd would be installed, but
it might not be in small chroots. It is a small library, so I hope this is not
a big issue.